1. Technical Field of the Invention
The invention relates to a resonator for electromagnetic waves with a stabilizer for stabilizing the effective length of the resonator. In addition, the invention relates to a method for stabilizing the length of a resonator.
2. Description of Related Art
A resonator for electromagnetic waves is, for example, an optical wave guide which forms part of a fiber laser. Fiber lasers are known in the art, for example, from X. Shan et al., "Stabilizing Er Fiber Soliton Laser With Pulse Phase Locking", Electronics Letters, 16 Jan. 1992, Vol. 28, No. 2, pages 182 to 184. Described therein is an active mode coupled fiber laser. The fiber laser is constructed as a ring laser and serves as an optical pulse generator. Into the fiber ring there is inserted, among others, a piezo-electric element, a phase modulator, and a coupler for coupling out a portion of the light. By applying the voltage to the piezo-electric element, the fiber segment around which the piezo-electric element is wound can be stretched; in this way, the length of the fiber segment can be changed. A portion of the light from the fiber ring is coupled out by the coupler and conveyed to a phase control device which generates the voltage for the piezo-electric element.
It is noted in the aforementioned publication that fiber lasers can be very sensitive to changes in temperature, i.e. the length of the fiber ring can change as a result of, e.g., temperature variations which can lead to an unstable laser operation. This poses a problem in particular in data transmission technology where minute instabilities can already cause unacceptable bit error rates. The piezo-electric element is capable of reducing the effect of a temperature dependent change in length, because the fiber segment can be stretched more or less depending on the magnitude and the direction of the change in length.
In this way, the length, or more accurately, the effective lengths of the fiber ring (resonator) is stabilized. For the effective lengths L.sub.eff holds: L.sub.eff =.sqroot..sub.68 .times.L.sub.geo, with the dielectric constant .epsilon. and the geometrical length L.sub.geo ; for optical applications, .sqroot..epsilon. is equal to the index of refraction n and the term "optical length" is used.
As an alternative to the aforedescribed concept, the optical length of a fiber segment can also be stabilized by controlling the temperature of the fiber segment. The required accuracy of the control of about .+-.0.01.degree. C. requires, however, a substantial technical investment.